A change in velocity is called acceleration. It can be positive or negative, though negative acceleration, or a decrease in velocity, is often referred to as deceleration.
Acceleration is the rate of change in velocity of an object over time. It is the net result of all the forces acting upon the object. The application of a physical force, such as hitting a ball with a bat, causes it to accelerate, as long as the force is sufficient to overcome the inertia of the object. A falling object accelerates at a constant rate due to the force of the Earth's gravity, until the force of gravity is matched by the force of air resistance. At that point, terminal velocity is reached. The object continues falling at a constant speed, and its acceleration is considered to be zero.